1. Technical Field
The present invention relates generally to waterjet systems and, in particular, to abrasive waterjet systems having a magnetically retained mixing tube.
2. Description of the Related Art
Conventional waterjet systems are used to process workpieces by pressurizing fluid and then delivering the pressurized fluid against a workpiece. Abrasive waterjet systems produce high-pressure abrasive fluid jets suitable for cutting through hard materials. High-pressure fluid can flow through a jewel orifice in a cutting head assembly to form a high-pressure fluid jet into which abrasive particles are entrained. This entrainment can take place within a chamber of the cutting head assembly. The high-pressure abrasive fluid jet passes through a mixing tube and is discharged from the mixing tube towards the workpiece.
The axis of the mixing tube has to be aligned with the waterjet coming out of the jewel orifice such that the abrasive fluid jet is properly aligned within the mixing tube. Conventional cutting head assemblies include mechanical components (e.g., collets, bushings, wedging devices, or nut assemblies) for installation of the mixing tube. High torques may be applied to these mechanical components which may require manual operation and result in losing accurate positioning of the mixing tube tip. Also, tools may be needed to access and to operate the mechanical components.
Collets are one type of mechanical component for retaining mixing tubes. If the cutting head assembly has a collet, a tapered surface must be precisely machined into the cutting head body to accommodate the collet, further increasing manufacturing costs. It may be difficult to remove the collet because the collet and the cutting head body may lock together, especially when the tapered surfaces of the cutting head body react significant forces (e.g., clamp-up forces). A hammer tapping process may therefore be needed to dislodge and to separate the collet from the cutting head body.
When the fluid jet passes through the mixing tube at a high velocity, the mixing tube, even if made of a highly wear-resistant material, experiences appreciable wear along its interior cylindrical surface surrounding the fluid jet. Accordingly, mixing tubes have to be replaced periodically within a time as short as a half hour, or perhaps as long as 100 hours, depending upon the material forming the mixing tube, as well as other factors, such as the types of entrained abrasive, working pressures, flow rates, etc. Frequent replacement of worn mixing tubes often leads to problems attributable to the way the mixing tube is retained in the cutting head body, resulting in impaired performance of the system.
Corrosion of the cutting head assembly may also impair performance. Components for retaining the mixing tube, for example, are often made of a material susceptible to corrosion, and have to be frequently replaced if exposed to corrosive materials for significant amounts of time. Replacing corroded components often causes damage to other components of the cutting head requiring replacement of non-corroded components. Water is one corrosive material that may lead to rusting of such components. Rust-resistant components, such as collets made entirely of stainless steel, are relatively expensive. Some cutting head assemblies use plastic type collets to lock the mixing tube and also to seal the mixing chamber.
Other types of abrasive waterjet systems include a removable mixing tube incorporated into a cartridge assembly. U.S. Pat. No. 5,144,766 discloses inserting a mixing tube and a jewel orifice into a housing of a cartridge. To replace the mixing tube, the seal disengages a cartridge housing of the cartridge assembly and may therefore result in contamination of the seal and the cartridge housing. This contamination can lead to leakage during operation of the waterjet system.